1. (Field of the Invention)
The present invention generally relates to a device for detecting the angle of rotation utilizable with various equipments and, more particularly, to the rotation angle detecting device for use in controlling the rotation of, for example, a compact drive motor or for use in detecting the angle of rotation for eventually detecting the position of, for example, a business machine. The present invention also relates to a bearing assembly utilizing such rotation angle detecting device and to a magnetic sensor array circuit utilizable in such rotation angle detecting device.
2. (Description of the Prior Art)
The applicant of the present invention has suggested in, for example, the Japanese Laid-open Patent Publication No. 2004-37133, published Feb. 5, 2004, or the Published International Application WO2006/064687A1, a rotation angle detecting device which utilizes magnetic sensor arrays and is therefore capable of detecting the angle of rotation with high precision. This known rotation angle detecting device includes magnetic sensor arrays comprised of a plurality of arrayed magnetic sensor elements integrated on a sensor chip together with a signal amplifying circuit, an analog-to-digital converter circuit and a digital signal processing circuit, which sensor chip is arranged in face-to-face relation with a magnet mounted on a rotatable member. The magnet is of a kind having a magnetic anisotropy in a circumferential direction and, on the other hand, the four linear arrays of magnetic sensors are arranged on the sensor chip so as to assume respective positions represented by the four sides of the imaginary shape of a rectangle.
With the rotation angle detecting device of the known structure discussed above, each of the magnetic sensor arrays is utilized to detect a distribution of magnetic fields in a direction perpendicular to the sensor chip, and respective outputs from those magnetic sensor arrays are then fed to the digital signal processing circuit through the signal amplifying circuit and then through the analog-to-digital (A/D) converter circuit so that the digital signal processing circuit can detect the zero-crossing-position corresponding to the NS boundary line of the magnetic field distribution on each of the magnetic sensor arrays to thereby detect the angle of rotation of the magnet.
In this case, since the size of the sensor chip is affected by the size of the magnetic sensor arrays, the rotation angle detecting device in its entirety can be compactized if the magnetic sensor arrays have their sizes reduced. Also, if the size of the magnetic sensor arrays is reduced, the surface area of the sensor chip can be reduced, bringing about an effect of reducing the cost of manufacture.
In his work, “Split-drain MOSFET Magnetic Sensor Arrays”, published in “Sensors and Actuators”, A24 (1990), pp. 107-116, James J. Clark describes the method of detecting the distribution of magnetic field by means of the magnetic sensor elements, MAGFETs, arranged in a matrix. He also describes the use of a read-out circuit in the form of a simplified voltage converting circuit, pointing out that the offset variation is remarkable in sensor output.
In the case of the rotation angle detecting device suggested by the applicant of the present invention, the smaller the length of time required for processing the sensor signals, which includes scanning the sensor signals, reading out them from the magnetic sensor arrays, and outputting the result of calculation of the angle by means of the digital signal processing circuit, the faster the angle can be detected, and the output delay time can therefore be reduced. Reduction of the output delay time is particularly desired for in detecting the angle of rotation of an object rotating at a high speed, detecting the angle of rotation in a control device requiring a high speed response and so on.
The reduction of the output time tends to be limited by the length of time required to read out the sensor signals and the length of time required for the calculating process of the rotation angle performed by the digital signal processing circuit. In particular, the length of time required to read out the sensor signals tends to increase with increase of the number of the magnetic sensor elements arranged to form the magnetic sensor arrays. Moreover, since the increased number of the magnetic sensor elements leads to increase of the number of data extracted to determine the zero-crossing-position by means of the digital signal processing circuit, the length of time required to complete the calculating process of the rotation angle is increased. In summary, if the number of the magnetic sensor elements forming the magnetic sensor arrays increases, the delay in time subsequent to the reading out of the sensor signals and before outputting the result of detection of the rotational angle will become large. Although the delay time referred to above may be reduced if the number of the magnetic sensor elements is small, the accuracy of detection of the rotation angle will be conversely lowered.
Also, since the accuracy of detection of the angle depends on the distance between two zero-crossing-positions detected on the magnetic sensor arrays, there is a problem in that the detection accuracy will be lowered if the distance between the two zero-crossing-positions is reduced as a result of downsizing of the magnetic sensor arrays as discussed above.
As a means to solve this problem, it may be contemplated, for example, to integrate the sensor signals read out from the magnetic sensor arrays or to calculate a mean value of the sensor signals, but this leads to reduction of the detection rate of the angle.
In view of the foregoing, a technology effective to reduce the cost of manufacturing by downsizing the sensor chip is desired, which does not lower the detecting speed, which is effective to keep a high detecting accuracy.
According to the Published International Application WO2006/064687A1 referred to above, where the angle of rotation of the magnet is to be determined in reference to the outputs from the magnetic sensor arrays, there is a problem in that since an offset variation exists among those respective outputs from the magnetic sensor elements, the accuracy of detection of the rotation angle tends to be reduced. In order to alleviate this problem, the applicant of the present invention has suggested that in the magnetic sensor arrays utilizing the magnetic sensor elements, MAGFETs, the offset variation can be reduced when the magnetic sensor elements are connected in parallel.
In the case of this technology of reducing the offset variation, although when four to eight magnetic sensor elements are connected in parallel, the offset variation can be reduced to a value of about ½ to about ⅓, the offset variation has to be further reduced in order to secure a sufficient accuracy of detection of the rotation angle.
On the other hand, in the sensor chip made of semiconductor, change in characteristic with change in environment such as, for example, temperature is inevitable. In other words, in the case of the sensor unit of the structure described above, the sensor output signal drifts with change in environment and the signal read-out circuit is also adversely affected by the change in environment and, therefore, there is a problem in that the accuracy of detection of the rotation angle eventually obtained tends to be lowered.
For those problems, the reduction is desired for by improving a method of driving the sensors and a circuit configuration of the read-out circuit. As a problem to be resolved, there is the need to reduce further the inherent offset variation of the sensors and to realize a circuit configuration for reading out the sensor signals without being adversely affected by the change in environment.
As far as the circuit configuration is concerned, the Japanese Patent Application No. 2005-167127 suggests the use of a reference sensor element short-circuiting between two output terminals thereof, storing as a reference offset value the sensor signal read out of the selected reference sensor element, and subtracting the stored reference offset value from the output of other magnetic sensor elements.
According to the suggested circuit configuration, since the reference sensor element is free from offset since the two output terminals of the magnetic sensor element are short-circuited, only an offset error, generated internally in the read-out circuit comprised of an amplifying circuit and others is extracted and appears in the output. This offset error is stored as the reference offset value and the offset value is subtracted from the output of the other magnetic sensor element. Accordingly, even though the respective circuit statuses of the magnetic sensor element and the magnetic sensor array circuit change with change in, for example, temperature, the charge of the circuit statuses can be corrected by utilizing the offset error of the circuit occasionally measured as described above. For this reason, the stable output signal that is hardly affected by the environment can be obtained.
However, since a magnetic sensor element forming a part of the magnetic sensor array is used as the dedicated reference sensor, there is a problem in that the number of the magnetic sensor element that are actually used for the detection is reduced.